In resistance welding, heat and pressure are used to fuse metals together. Heat generated by the resistance of the work pieces to the flow of electricity either melts the material at the interface, or at least reduces its strength to a level where the surfaces of the materials become plastic. When the flow of current stops, the electrode force is maintained, for a fraction of a second, while the weld rapidly cools and solidifies.
Broadly speaking, there are several parameters which can be controlled in the resistance welding process, these being the electrode force on the workpiece, the magnitude of the weld current, and the duration of the weld current. These parameters have different optimum values for different materials being welded. If these parameters are not closely controlled, the welds obtained can vary in quality.
Maintaining weld quality in integrated circuit packages and micro-circuit components and assemblies is particularly important. Precision welding equipment is used for the welding of these extremely small items. The weld heads for most prior art precision welding equipment are driven manually or pneumatically through a spring nest. Because of the limitations in control of these types of systems, a number of desirable control features are either very difficult or impossible to obtain.
With such prior art welding equipment there is no practical way to program in different speeds, forces or positions. Also, there is no satisfactory way to achieve the desired speed of response of the weld head to the onset of welding current or to prevent the force from continuing to increase after the weld has been completed, which can cause damage to the weld or the entire component.
Not only is there a problem with not being able to control the force with prior art welding equipment, additionally, there is no means for automatically controlling the speed and the duration of contact of the weld or bonding head and for varying the amount of force during contact. Therefore, there exists a need for the automatic control of speed, response time, force, duration and pressure of the weld head in a precision welding device for the welding of a broad range of precision applications such as integrated circuit packages and micro circuit components.